Encryption key generation

ABSTRACT

A first device transmits a first random number to a second device through a first quantum channel, and receives a second random number from the second device through a second quantum channel. The first device generates a first encryption key based on the first random number and the second random number. The second device transmits the second random number to the first device through the second quantum channel, and receives the first random number from the first device through the first quantum channel. The second device generates a second encryption key based on the first random number and the second random number.

REFERENCE TO RELATED APPLICATION

The present application is a Continuation application of Ser. 15/502,245filed on Feb. 7, 2017, which is a National Stage Entry ofPCT/JP2015/004158 filed on Aug. 20, 2015, which claims priority fromJapanese Patent Application 2014-170087 filed on Aug. 25, 2014, thecontents of all of which are incorporated herein by reference, in theirentirety.

TECHNICAL FIELD

The present invention relates to an information communication system, aninformation communication method and device to transmit and receiveinformation between communication devices.

BACKGROUND ART

In a data transmission between communication devices, not all ofinformation transmitted from a transmission side is received at areceiving end.

For example, it is known that a packet loss occurs due to a load stateor the like of a network, and, in addition, there is a communicationsystem in which only a part of transmitted data reaches to the receiveras characteristics of a transmission system including a transmitter, areceiver, and a channel that connects them. As an example of suchcommunication system, a quantum key distribution (QKD) system will bedescribed briefly.

It is necessary to share a shared key required for encryption anddecryption of information between a transmission end and a receiving endas secret information, and QKD technology is regarded to be promising asa technology to generate and share such secret information. According tothe QKD technology, contrary to a conventional optical communication, itis possible to generate and share a common key between a transmitter anda receiver by transmitting a random number with the number of photonsper bit equal to one or less. The QKD technology has the security thatis based on the principle of quantum mechanics that a photon observedonce cannot be completely returned to the quantum state before theobservation, not the security that is based on conventionalcomputational complexity.

It is necessary in the QKD technology to carry out several steps beforean encryption key used for cryptographic communication is generated.Hereinafter, a generation process of a typical encryption key will bedescribed with reference to FIG. 1.

As shown in FIG. 1, in a single photon transmission, a random number istransmitted through a quantum channel by a weak optical pulse train withthe number of photons per bit equal to one or less, as mentioned above.As the QKD method, a BB84 method using four quantum states is widelyknown (Non Patent Literature 1), for example. When a transmittertransmits an original random number by a single photon transmission,most of it is lost due to the loss or the like of a transmission line;and bits that can be received by a receiver become a very small part ofthe transmitted bits, which is called a raw key. For example, the datavolume that can be received by a receiver is about 1/1000 of thetransmitted data volume.

Subsequently, a basis reconciliation, error correction, and privacyamplification processing are performed on the raw key that is receivedwith most of the transmitted random numbers having been lost due to thequantum channel transmission, using a communication channel with normaloptical intensity (classical channel). In each step of the basisreconciliation, error correction and, privacy amplification processing,a bit elimination is carried out to eliminate bits disclosed to theother side and the possibility of wiretapping. Thus, in a transmissionsystem in which most of transmitting data is lost in a transmissionchannel, and data elimination is performed in subsequent processes, areceived data volume finally obtained becomes very small compared withthe transmitted data volume.

CITATION LIST Non Patent Literature

[NPL 1] “QUANTUM CRYPTOGRAPHY, PUBLIC KEY DISTRIBUTION AND COIN TOSSING”IEEE Int. Conf. on Computers, Systems, and Signal Processing, Bangalore,India, Dec. 10-12, 1984, pp. 175-179, Bennett, Brassard

SUMMARY OF INVENTION Technical Problem

As mentioned above, in a transmission system in which most oftransmitting data is lost, a problem newly arises that the processingefficiency declines because large unbalance occurs with respect to adata volume to be processed between a transmission end to processtransmitting data and a receiving end to process received data, andbecause the processing load of the transmitting end becomes larger.

The object of the present invention is to provide an informationcommunication system, an information communication method and devicethat can achieve the dispersion of a processing load betweencommunication devices that perform information transmission.

Solution to Problem

An information communication system according to an exemplary aspect ofthe present invention, an information communication system to transmitand receive information between communication devices, includes a firsttransmission system configured to transmit information in a directionfrom a first communication device to a second communication device; anda second transmission system configured to transmit information in adirection opposite to the direction of the first transmission system,wherein part of transmission information is received as receivedinformation in each of the first transmission system and the secondtransmission system.

A communication device according to an exemplary aspect of the presentinvention, a communication device to transmit and receive information toand from another communication device, includes a transmitting means fortransmitting information to the another communication device through afirst transmission line; and a receiving means for receiving informationthrough a second transmission line from the another communicationdevice, wherein part of transmission information is received as receivedinformation in each of the first transmission line and the secondtransmission line.

An information communication method according to an exemplary aspect ofthe present invention, an information communication method to transmitand receive information between the communication devices, includestransmitting and receiving information at each of a first communicationdevice and a second communication device by use of a first transmissionsystem and a second transmission system, the first transmission systemand the second transmission system having transmission directionsopposite to each other; and receiving part of transmission informationas received information in each of the first transmission system and thesecond transmission system.

Advantageous Effects of Invention

According to the present invention, it becomes possible to disperse aprocessing load between communication devices.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram to illustrate information-processing stepsin a quantum key distribution (QKD) system.

FIG. 2 is a block diagram illustrating a schematic configuration of aninformation communication system in accordance with a first exampleembodiment of the present invention.

FIG. 3 is a block diagram illustrating a configuration example of aninformation communication system in accordance with a second exampleembodiment of the present invention.

FIG. 4 is a block diagram illustrating a configuration example of aninformation communication system in accordance with a third exampleembodiment of the present invention.

EXAMPLE EMBODIMENT The Outline of Example Embodiments

According to the example embodiments of the present invention, when partof transmission information is received as received information in atransmission system set between communication devices, it becomespossible to disperse a processing load between the communication devicesby providing a pair of transmission systems with the transmissiondirections opposite to each other. In each communication device, if apredetermined processing using transmission information and apredetermined processing using receiving information are performed, theequalization of the processing loads can be achieved between thecommunication devices, and sufficient information generation efficiencycan be obtained. Because both of the transmitter and the receiver areprovided in each communication device, transmit data can be received bya receiver in the own device, and it becomes possible to adjustparameters of a transmitter in each communication device. Exampleembodiments of the present invention will be described below in detailusing figures. The direction of the arrow in the figures indicates adirection as an example and does not limit the direction of the signalsbetween the blocks.

1. The First Example Embodiment

As illustrated in FIG. 2, in an information communication systemaccording to the first example embodiment of the present invention, afirst communication device 10 and a second communication device 20perform information transmission in the directions opposite to eachother by a first transmission system 31 and a second transmission system32. The first transmission system 31 performs one direction transmissionfrom the first communication device 10 to the second communicationdevice 20, and includes a transmitter 101 of the first communicationdevice 10, a receiver 201 of the second communication device 20, and afirst transmission line 33 connecting the transmitter 101 and thereceiver 201. The second transmission system 32 performs one directiontransmission in the direction opposite to that of the first transmissionsystem 31, and includes a receiver 102 of the first communication device10, a transmitter 202 of the second communication device 20, and asecond transmission line 34 connecting the transmitter 202 and thereceiver 102.

The first communication device 10 includes the transmitter 101, thereceiver 102 and a data processor 103. The data processor 103 receivesinputs of transmission information TD1 on the transmitter 101 andreceived information RD2 from the second communication device 20 that isreceived by the receiver 102, and performs predetermined data processingon the information respectively. The second communication device 20includes the receiver 201, the transmitter 202 and, a data processor203. The data processor 203 receives inputs of transmission informationTD2 on the transmitter 202 and received information RD1 from the firstcommunication device 10 that is received by the receiver 201, andperforms predetermined data processing on the information respectively.The data processor 103 of the first communication device 10 and the dataprocessor 203 of the second communication device 20 can perform anidentical information processing and generate a similar sort ofinformation, for example.

The first transmission system 31 transmits the information in thedirection from the first communication device 10 to the secondcommunication device 20, and has the characteristics that a receivedinformation volume becomes less than a transmission information volume.That is to say, the transmission information TD1 transmitted from thetransmitter 101 is partially lost in the first transmission line 33and/or the receiver 201, and only part of the transmission informationTD1 is received by the receiver 201 as the received information RD1.

The second transmission system 32 transmits the information in thedirection from the second communication device 20 to the firstcommunication device 10 contrary to the first transmission system 31,and has the characteristics that a received information volume becomesless than a transmission information volume, as is the case with thefirst transmission system 31. That is to say, the transmissioninformation TD2 transmitted from the transmitter 202 is transmittedthrough the second transmission line 34 and is received by the receiver102. On this occasion, the transmission information TD2 is partiallylost in the second transmission line 34 and/or the receiver 102, andonly part of the transmission information TD2 is received by thereceiver 102 as the received information RD2.

Consequently, the data processor 103 receives inputs of the transmissioninformation TD1 having a large data volume and the received informationRD2 having a relatively small data volume and performs processing, andsimilarly, the data processor 203 receives inputs of the transmissioninformation TD2 having a large data volume and the received informationRD1 having a relatively small data volume and performs processing. Ifthe first transmission system 31 and 32 have similar transmissioncharacteristics, and the data processors 103 and 203 perform identicalinformation processing, it becomes possible to reduce the unbalance ofloads regarding the data processing between the first communicationdevice 10 and the second communication device 20.

As mentioned above, according to the present example embodiment, itbecomes possible to disperse the processing loads between thecommunication devices by setting a pair of transmission systems 31 and32 each of which transmits in a direction opposite to each other. Thatis to say, the processing capacity can be utilized efficiently becausethe processing load can be equalized between the communication devices.It becomes possible to generate efficiently desired information becauseeach communication device can generate the information by processingboth of the transmission information and the received information.

2. The Second Example Embodiment

As illustrated in FIG. 3, an information communication system accordingto the second example embodiment of the present invention is a system inwhich the first example embodiment mentioned above is applied to a QKDsystem.

In FIG. 3, a communication device A and a communication device Btransmit a single photon pulse train modulated by random numberinformation in the directions opposite to each other using a quantumchannel transmission system Q1 and a quantum channel transmission systemQ2. The quantum channel transmission system Q1 includes a transmitter301 of the communication device A, a receiver 401 of the communicationdevice B, a transmission line (quantum channel) connecting thetransmitter 301 and the receiver 401. The quantum channel transmissionsystem Q2 includes a transmitter 402 of the communication device B, areceiver 302 of the communication device A, and a transmission line(quantum channel) connecting the transmitter 402 and the receiver 302.In the present example embodiment, respective transmission lines of thequantum channel transmission systems Q1 and Q2 may be composed ofoptical fibers that physically differ from each other or may bewavelength-multiplexed in an identical optical fiber.

The communication device A and the communication device B performoptical communication with the optical power having a normal level usinga classical channel transmission system C. The classical channeltransmission system C includes an optical communication unit 304 of thecommunication device A, an optical communication unit 404 of thecommunication device B, and a transmission line (classical channel)connecting the optical communication unit 304 and the opticalcommunication unit 404. The communication device A and the communicationdevice B perform, in addition to the synchronous processing, the basisreconciliation with the other communication device, the errorcorrection, and the privacy amplification processing, as mentionedabove, using the classical channel transmission system C. A classicalchannel in the classical channel transmission system C may be providedby wavelength multiplexing in the same optical fiber as that includingthe quantum channel transmission systems Q1 and Q2. Alternatively, asynchronization channel for the synchronous processing can be providedin another optical fiber.

The classical channel of the classical channel transmission system C maybe an electric communication path by an electric signal, not an opticalcommunication. In this case, it is only necessary to replace the opticalcommunication units 304 and 404 with communication units that transmitand receive an electric signal.

The communication device A includes the transmitter 301, the receiver302, an encryption key generation unit 303, the optical communicationunit 304, and a control unit 305. The encryption key generation unit 303corresponds to the data processor 103 in the first example embodiment.The encryption key generation unit 303 receives inputs of transmissioninformation (original random number) TD1 on the transmitter 301 andreceived information RD2 received by the receiver 302 from thecommunication device B. Then, the encryption key generation unit 303generates an encryption key by performing the basis reconciliation withthe communication device B through the optical communication unit 304,the error correction, and the privacy amplification processing, asmentioned above. The control unit 305 controls the overall operations ofthe communication device A.

The basic configuration of the communication device B is similar to thatof the communication device A. That is to say, the communication deviceB includes the receiver 401, the transmitter 402, an encryption keygeneration unit 403, the optical communication unit 404, and a controlunit 405. The encryption key generation unit 403 corresponds to the dataprocessor 203 in the first example embodiment. The encryption keygeneration unit 403 receives inputs of transmission information(original random number) TD2 on the transmitter 402 and receivedinformation RD1 received by the receiver 401 from the communicationdevice A. Then, the encryption key generation unit 403 generates anencryption key by performing the basis reconciliation with thecommunication device A through the optical communication unit 404, theerror correction, and the privacy amplification processing, as mentionedabove. The control unit 405 controls the overall operations of thecommunication device B.

In the quantum channel transmission system Q1, the transmitter 301 ofthe communication device A puts the transmission information (originalrandom number bit information) TD1 on a very weak optical pulse trainwith the number of photons per bit equal to one or less, and transmitsit to the receiver 401 of the communication device B through a quantumchannel. The weak optical pulse train in transmission is lost in themiddle of the transmission line, and only part of it reaches thereceiver 401. The receiver 401 outputs detected data to the encryptionkey generation unit 403 as received information RD1. As mentioned above,the information volume of the received information RD1 gets down toabout 1/1000 of the information volume of the transmission informationTD1, for example.

In the quantum channel transmission system Q2, the transmitter 402 ofthe communication device B puts the transmission information (originalrandom number bit information) TD2 on a very weak optical pulse trainwith the number of photons per bit equal to one or less, and transmitsit to the receiver 302 of the communication device A through a quantumchannel. In this case, its transmission direction is opposite to that ofthe quantum channel transmission system Q1. The weak optical pulse trainin transmission is lost in the middle of the transmission line, and onlypart of it reaches the receiver 302. The receiver 302 outputs detecteddata to the encryption key generation unit 303 as received informationRD2. It is assumed that the information volume of the receivedinformation RD2 also gets down to the same level (about 1/1000) of theinformation volume of the transmission information TD2 as is the casewith the quantum channel transmission system Q1.

The encryption key generation unit 303 receives inputs of thetransmission information TD1 having a large data volume and the receivedinformation RD2 having a quite small data volume. The encryption keygeneration unit 303 can generate a first encryption key by performingthe basis reconciliation, the error correction, and the privacyamplification processing, on the transmission information TD1 and thereceived information RD1 in the other communication device B through theclassical channel transmission system C. Similarly, the encryption keygeneration unit 403 also receives inputs of the transmission informationTD2 having a large data volume and the received information RD1 having aquite small data volume. The encryption key generation unit 403 cangenerate a second encryption key by performing the basis reconciliation,the error correction, and the privacy amplification processing, on thetransmission information TD2 and the received information RD2 in theother communication device A through the classical channel transmissionsystem C. Because information volume attenuation arises equally in eachof a pair of quantum channel transmission systems Q1 and Q2 havingtransmission directions opposite to each other, the same level ofinformation volume is processed; consequently, the equalization ofprocessing loads can be achieved between the encryption key generationunits 303 and 403.

3. The Third Example Embodiment

An information communication system according to the third exampleembodiment of the present invention is a system obtained by adding aself-diagnostic function to each communication device according to theabove-mentioned second example embodiment. Specifically, a transmissionparameter adjusting function, and an optical route switching function ofchanging the route of transmission light so as to input the transmissionlight into the receiver in the own device at a time of parameteradjustment mode, are added.

Generally, in order to adjust a parameter such as transmission opticalintensity of a transmitter that transmits the above-mentioned weakoptical pulse, a receiver to receive the weak optical pulse is required.Since weak light is very weak light with one photon or less per bit, adetector that can detect a single photon is required; consequently, anavalanche photodiode is usually used. Accordingly, the parameteradjustment is performed using a receiver in the other communicationdevice.

However, there is likely to be a wire-tapper on the transmission path,and there is the threat of damaging the security of QKD if a wire-tapperintervenes during the parameter adjustment. In addition, since a singlephoton detector is very expensive, it is not rational to install thereceiver only for the parameter adjustment.

According to the present example embodiment, each communication devicehas a transmitter and a receiver for a quantum channel because a pair ofquantum channel transmission systems Q1 and Q2 for transmission inopposite direction is provided. Accordingly, it is possible to utilizethis receiver as a single photon receiver for parameter adjustment. Thepresent example embodiment will be described below with reference toFIG. 4. In FIG. 4, the encryption key generation unit and the opticalcommunication unit are not illustrated that are included in thecommunication device according to the above-mentioned second exampleembodiment.

As illustrated in FIG. 4, the communication device A according to thepresent example embodiment includes an optical switch 311 in the outputside of the transmitter 301 and an optical switch 312 in the receivingside of the receiver 302, respectively. In addition, the communicationdevice A includes a parameter adjustment unit 313 that adjusts aparameter such as the transmission optical intensity of the transmitter301 using the detected data by the receiver 302. The control unit 305controls the switching operations of the optical switches 311 and 312and the adjustment operations of the parameter adjustment unit 313.Similarly, the communication device B according to the present exampleembodiment includes an optical switch 412 in the output side of thetransmitter 402, and an optical switch 411 in the receiving side of thereceiver 401, respectively. In addition, the communication device Bincludes a parameter adjustment unit 413 that adjusts a parameter suchas the transmission optical intensity of the transmitter 402 using thedetected data by the receiver 401. The control unit 405 controls theswitching operations of the optical switches 411 and 412 and theadjustment operations of the parameter adjustment unit 413.

The optical switch 311 in the communication device A includes an inputport Pi, an output ports Po1 and Po2, and the optical switch 312includes input ports Pi1, Pi2 and an output port Po. The input port Piof the optical switch 311 is optically connected to the output of thetransmitter 301, the output port Po1 is optically connected to theabove-mentioned quantum channel transmission system Q1, and the outputport Po2 is optically connected to the input port Pi2 of the opticalswitch 312, respectively. The output port Po of the optical switch 312is optically connected to the input of the receiver 302, the input portPi1 is optically connected to the above-mentioned quantum channeltransmission system Q2, and the input port Pi2 is optically connected tothe output port Po2 of the optical switch 311, respectively.

In a normal operation state, the control unit 305 sets the opticalswitch 311 and the optical switch 312 so that the input port Pi and theoutput port Po1 of the optical switch 311 may be connected, and theinput port Pi1 and the output port Po of the optical switch 312 may beconnected. Consequently, the operation for the encryption key generationis performed through the quantum channel transmission systems Q1 and Q2,as mentioned above.

At the time of parameter adjustment, the control unit 305 sets theoptical switch 311 and the optical switch 312 so that the input port Piand the output port Po2 of the optical switch 311 may be connected, andthe input port Pi2 and the output port Po of the optical switch 312 maybe connected. Consequently, the weak optical signal outputted from thetransmitter 301 is inputted into the receiver 302 through the outputport Po2 of the optical switch 311, and the input port Pi2 and theoutput port Po of the optical switch 312. This enables the parameteradjustment unit 313 to adjust a parameter such as the transmissionoptical intensity of the transmitter 301 using the detected data by thereceiver 302. In the communication device B, the optical switches 411and 412, are configured and operate as with the above; accordingly, thedescription of them is omitted.

As mentioned above, according to the present example embodiment, anoptical switch is included in each communication device as optical routeswitching means for turning back the transmission light from thetransmitter to the receiver in the own device. At the time of theparameter adjustment mode, the control unit can complete the parameteradjustment in the own device by switching the optical switches so thatthe transmission light may be inputted into the receiver in the owndevice; therefore, it is possible to perform the parameter adjustment ofthe transmitter without damaging the security of QKD.

In the first to third example embodiments mentioned above, a pair oftransmission system having transmission directions opposite to eachother has been illustrated, but the present invention is not limited tothese example embodiments, and a communication system having a pluralityof pairs of transmission systems may be used.

The present invention has been described using the above-mentionedexample embodiments as exemplary examples. However, the presentinvention is not limited to the above-mentioned example embodiments.That is to say, in the present invention, various aspects that a personskilled in the art can understand can be applied within the scope of thepresent invention.

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2014-170087 filed on Aug. 25, 2014, thedisclosure of which is incorporated herein in its entirety by reference.

INDUSTRIAL APPLICABILITY

The present invention is generally applicable in an informationcommunication system in which information transmission is performed by aplurality of transmission systems each of which has a predeterminedtransmission direction.

REFERENCE SIGNS LIST

-   -   10 First communication device    -   20 Second communication device    -   31 First transmission system    -   32 Second transmission system    -   33 First transmission line    -   34 Second transmission line    -   101 Transmitter    -   102 Receiver    -   103 Data processor    -   201 Receiver    -   202 Transmitter    -   203 Data processor    -   301 Transmitter    -   302 Receiver    -   303 Encryption key generation unit    -   304 Optical communication unit    -   305 Control unit    -   311, 312 Optical switch    -   313 Parameter adjustment unit    -   401 Receiver    -   402 Transmitter    -   403 Encryption key generation unit    -   404 Optical communication unit    -   405 Control unit    -   41, 412 Optical switch    -   413 Parameter adjustment unit

1. A device comprising: a transmitter configured to transmit a firstrandom number to another device through a first quantum channel, areceiver configured to receive a second random number from the anotherdevice through a second quantum channel; and a data processor configuredto generate an encryption key based on the first random number and thesecond random number.
 2. The device according to claim 1, furthercomprising: a communication unit configured to communicate with theanother device and to perform at least one of synchronization, basesreconciliation with the another device, error correction and privacyamplification.
 3. The device according to claim 1, wherein the firstquantum channel and the second quantum channel are configured to use thesame optical cable.
 4. The device according to claim 1, wherein thefirst quantum channel and the second quantum channel are configured touse the different optical cables.
 5. The device according to claim 2,wherein the communication unit is configured to use an electrical cable.6. The device according to claim 1, wherein the first quantum channeland the second quantum channel are configured to use weak opticalpulses.
 7. A system comprising: a first device configured to: transmit afirst random number to a second device through a first quantum channel,receive a second random number from the second device through a secondquantum channel, and generate a first encryption key based on the firstrandom number and the second random number; and the second deviceconfigured to: transmit the second random number to the first devicethrough the second quantum channel, receive the first random number fromthe first device through the first quantum channel, and generate asecond encryption key based on the first random number and the secondrandom number.
 8. The system according to claim 7, wherein the firstdevice is further configured to communicate with the second device andto perform at least one of synchronization, bases reconciliation withthe second device, error correction and privacy amplification, and thesecond device is further configured to communicate with the first deviceand to perform at least one of synchronization, bases reconciliationwith the first device, error correction and privacy amplification. 9.The system according to claim 7, wherein the first quantum channel andthe second quantum channel are configured to use the same optical cable.10. The system according to claim 7, wherein the first quantum channeland the second quantum channel are configured to use the differentoptical cables.
 11. The system according to claim 8, wherein the firstdevice is configured to communicate with the second device by using anelectrical cable, and the second device is configured to communicatewith the first device by using an electrical cable.
 12. The systemaccording to claim 7, wherein the first quantum channel and the secondquantum channel use weak optical pulses.
 13. A method comprising:transmitting a first random number to another device through a firstquantum channel, receiving a second random number from the anotherdevice through a second quantum channel; and generating an encryptionkey based on the first random number and the second random number. 14.The method according to claim 13, further comprising: communicating withthe another device, and performing at least one of synchronization,bases reconciliation with the another device, error correction andprivacy amplification.
 15. The method according to claim 13, wherein thefirst quantum channel and the second quantum channel use the sameoptical cable.
 16. The method according to claim 13, wherein the firstquantum channel and the second quantum channel use the different opticalcables.
 17. The method according to claim 14, wherein the communicatingcommunicates with the another device by using an electrical cable. 18.The method according to claim 13, wherein the first quantum channel andthe second quantum channel use weak optical pulses.
 19. A methodcomprising: in a first device, transmitting a first random number to asecond device through a first quantum channel, receiving a second randomnumber from the second device through a second quantum channel; andgenerating a first encryption key based on the first random number andthe second random number; and in the second device, transmitting thesecond random number to the first device through the second quantumchannel, receiving the first random number from the first device throughthe first quantum channel, and generating a second encryption key basedon the first random number and the second random number.
 20. The methodaccording to claim 19, further comprising: in the first device,communicating with the second device, and performing at least one ofsynchronization, bases reconciliation with the second device, errorcorrection and privacy amplification; and in the second device,communicating with the first device, and performing at least one ofsynchronization, bases reconciliation with the first device, errorcorrection and privacy amplification.
 21. The method according to claim19, wherein the first quantum channel and the second quantum channel usethe same optical cable.
 22. The method according to claim 19, whereinthe first quantum channel and the second quantum channel use thedifferent optical cables.
 23. The method according to claim 20, whereinin the first device, the communicating with the second devicecommunicates with the second device by using an electrical cable, and inthe second device, the communicating with the first device communicateswith the first device by using an electrical cable.
 24. The methodaccording to claim 19, wherein the first quantum channel and the secondquantum channel use weak optical pulses.